1. Field of the Invention
The present invention relates to methods and apparatus for monitoring and controlling the composition of fluid mixtures and more particularly to a method and apparatus utilizing nearly simultaneous measurements derived from the velocity of sound in the separate components of these fluid mixtures in a comparative sensing technique.
Some industrial processes for producing electronic devices such as chemical vapor deposition, epitaxy, reactive ion etch, and dopant diffusion require precise fluid mixtures. At present, fluid mixtures are generated by blending controlled flows of pure fluids or fluid mixtures, and the individual flows are controlled by either mechanical or electronically actuated flow regulating devices. The present invention can be used to sense the fluid compositions resulting from mixing of controlled flows of the constituent fluids, and thus to monitor the operation of said flow regulating devices.
2. Description of the Prior Art
Prior to the present invention, the best state of the art in fluid composition control was achieved through the use of mass flow controllers. While such devices represent a significant improvement over previously used fluid flow control devices, they are plagued by inherent problems. The most significant of these problems arise from the fact that the sensing of fluid mass flow is achieved by heat transfer through a metallic capillary tube which can react with the corrosive fluids involved or utilized int he processes or can become coated with foreign substances in which case the sensing devices are prone to drift in their readings. This necessitates frequent removal of the sensing devices from the processes for cleaning and re-calibration. Another major problem is that the best state of the art sensing devices achieve a precision only about 0.5% of full rated flow.
The present invention vastly improves upon these limitations. The instrument disclosed and claimed is considerably more immune to corrosion, the sensing elements never contacting the process gases. The precision obtained is on the order of 0.01% of actual fluid composition for most fluid mixtures over a much more extensive range of 0% to 100% of any fluid component of the mixture. Still another advantage derives from the fact that the sound velocity for a stable fluid is a constant value, and for this reason the disclosed instrument of the present invention is designed to respond only to sound velocity and need not be re-calibrated against a standard to correct for drift.
Another use for the present invention involves different prior art. In those processes requiring the use of low vapor pressure substances, a carrier fluid is passed over a solid or liquid source of the substance which is held at a controlled temperature so as to establish a controlled vapor pressure and, presumably, controlled mole fractions of the constituents in the carrier fluid. However, rather than relying on this technique to determine mole fractions, there is a great need to be able to measure such mole fractions directly. This is a particular problem in those processes utilizing alkylmetals, or organic forms of such substances as arsenic, cadmium, gallium, indium, mercury, phosphorous, tellurium, zinc and other substances. At the present time, researchers involved with the development of gallium arsenide and indium phosphide device technology need continuous data on the mole fractions of substances actually delivered to their reactors.
Prior to the present invention, this need for data was either unfilled or a mass spectrometer was employed for chemical constituent analysis. The mass spectrometer depends on precisely controlled conditions of temperature, magnetic flux, vacuum, and voltage in order to achieve the controlled sampling rate, molecular fragmentation, and fragment classification which are required for reproducible data. All this requires complicated machinery vulnerable to malfunction. Additionally, quantitative analysis with mass spectroscopy is an empirical technique requiring periodic re-calibration. The present invention can be used to monitor the mole fractions of low vapor pressure substances in the carrier gas directly and thus represents a vast improvement in economy and ease of operation since it is relatively inexpensive, simple, rugged, and requires no vacuum, and no re-calibration.
Yet another use for the present invention involves still different prior art. In those processes requiring highly purified gases to be delivered at a reactor vessel, great care is taken to ensure against incursion of contaminating chemicals, such as the constituents in air, as the highly purified gases are piped from compressed gas or evaporation sources to the processing chambers. Microcontamination of these processes as a result of both air incursion and particulate generation has been directly related to defects in the semiconductor devices produced in these processes, and the sources of such microcontamination must be rigorously eliminated. This is a particular problem in the production of devices of sub-micron geometry. Prior to the present invention, air incursion monitoring has been done by sample extraction followed by analysis of the high purity gas for the presence of hydrocarbons, oxygen, and water with separate analyzers. Each one of these analyzers has its own maintenance and calibration requirements, and the sample extraction itself can cause contamination. The present invention can be used to monitor for such an incursion. It can be constructed of inert materials which will not react with the process fluids. It contains no parts subject to frictional wear and produces no measurable particulate contamination in the process fluid stream. In addition, the instrument of the present invention need not be taken off line or removed for re-calibration. These attributes represent a particular advantage for use with processes which are very sensitive to microcontamination.
Still another use for the present invention involves yet different prior art. Industrial processes such as cracking, catalysis, and alkylation require that the compositions of the input and output fluid streams be monitored and/or controlled. This is routinely performed by chemical analysis employing infrared absorption, chromatography, or mass spectroscopy. Process stream analysis is usually continuous and can be supplemented with laboratory analysis. The present invention cannot replace these other techniques which perform constituent analysis, but can supplement them when used to detect changes in the composition of process streams and can thus improve the reliability of control schemes based on constituent analysis by immediately alarming when the constituency of a process stream varies beyond preset limits indicating that a process analyzer has drifted or failed. The embodiment of the present invention can, in some cases, stand in for a process analyzer while it is being repaired or recalibrated and then return to the monitoring mode.
Yet a further use of the present invention involves still different prior art. Some industrial processes require monitoring for low concentrations of contaminants such as moisture or air. The present invention can be used to detect these impurities in process streams. It is of particular use in highly corrosive process streams such as those containing halogens and their corrosive derivatives since the entire body of the instrument of the present invention can be constructed of corrosion resistant material. In addition, calibration and verification of operation in the prior art instruments, such as hygrometers, present a major problem in malevolent processes streams such as nearly pure chlorine, hydrogen chloride, or ammonia because any contaminant introduced or entering the stream quickly combines with the process chemicals or adheres to surfaces and is not available in the process stream for verification of the detection capability of the instrument. For this reason, the prior art detection instrument must be removed from the process stream and calibrated off line, then returned to the process stream and there is no verification of detection capability in the actual malevolent operating environment. The present invention, not needing re-calibration, need not be removed from the process stream. This is a significant practical advantage because the removal itself is both hazardous and can contaminate the process stream.
Still a further use for the present invention involves yet different prior art. In many industries which utilize toxic, corrosive, explosive, pyrophoric or inert gases (which if leaked into the air in sufficient quantity can cause asphyxiation by oxygen deficiency), a basic problem in the production or testing of their products is the safety of the personnel and apparatus being used in the areas where these chemicals are employed. Safety can be assured and monitored by continuously sampling the air in those environments for the presence of even minute amounts of such gases. The computer chip industry is particularly concerned because it regularly utilizes such gases in the production of its products. In these environments, a detection instrument's primary asset is reliability. Prior to the present invention, these air monitoring tasks have been performed by a variety of means such as electrochemical-based sensors, catalytic combustion-based sensors, flame photometric detectors, chemically treated paper tape based monitors, mass spectroscopy, and chromatography. While these sensing techniques have been developed to a high state of the art, they all suffer from one important disadvantage: namely, the user can never be absolutely sure they are operational. The instrument of the present invention can be used to monitor these environments; its greatest advantages being its reliability and its freedom from the requirements for frequent re-calibration and testing. The invention's use of sound which is constantly being sent and received through fluid samples provides inherent and continuous end-to-end affirmation of the detection process; a very practical means thereby to assure the device is working, and to alert operators instantaneously if it is not working.
Yet another use of the present invention involves another class of fluids: namely liquids. Liquid fluid mixtures exhibit variations in sound velocity which are dependent on the composition of the fluid. However, temperature has an unpredictable effect on sound velocity in liquid mixtures. In some liquids sound velocity increases as temperature increases and in other liquids the opposite effect is observed. It is expected that analysis of liquid samples utilizing the acoustic design principles and data handling methodology disclosed herein will prove useful in specific applications where the capabilities of presently available instrumentation is deficient; for example, monitoring of boiler feed water for sudden contamination and monitoring the concentration of dissolved substances in water used to extract sugar from sugar beets or used to blanch french fried potatoes or used to prepare syrup for soft drink and fruit canning industries.
And yet a further use for he present invention involves still different prior art. Many industrial and laboratory processes involve fermentation. For example, penicillin and other antibiotics are so produced, as are monosodium glutamate and other amino acids and acetone, ethyl alcohol and other organic chemicals. In addition, new and experimental products are so produced by recombinant organisms. Fermentation is performed in the liquid phase, but many of the essential chemicals supplied or produced are gaseous. Measurement of the gases taken up and exuded during fermentation provides a valuable tool for monitoring and control. Fermentation processes are difficult to characterize and a variety of measurements is useful, one of the most important being oxygen concentration. When oxygen or another gas is sparged through the culture medium, a mass spectrometer or a gas chromatograph is frequently employed to analyze the top gas and aid in characterizing the process. Equipped with chemical getters to selectively and sequentially eliminate specific constituents from a sample, an instrument of the present invention may be used to sample the top gas and monitor for key parameters such as carbon dioxide, methane, ammonia, and other gases of interest, thereby tracking the state of the organisms and the fermentation process. Again, an instrument of the present invention is a vast improvement over both a gas chromatograph and a mass spectrometer in terms of simplicity, reliability, and freedom from the need for re-calibration.